Remove admin namespace

Offered for comment - it's not hugely useful, but does clean things up a little.

Publisher used to do more than it does now:

* At one stage frontend was actually a set of nested sinatra apps running within publisher
* Before we rolled out the content API it offered a JSON API to get edition information

Back then it made sense to nest all the publishing URLs under /admin, but with those responsibilities gone that just adds noise to the code and the URLs. This branch removes all of that.